1. Field of the Invention
The present invention broadly relates to a framework structure and, more particularly, to a joint for use in connecting a main pipe and an auxiliary pipe which in combination constitute the framework structure.
2. Description of the Prior Art
The simplest form of conventional framework structure includes, as shown in FIG. 1, two main pipes 1, 3 and auxiliary pipes 5, 7 connected between two main pipes by welding. In the production of this conventional structure, when the main pipes 1 and 3 have a circular cross-section, it is necessary to cut and machine the end surfaces of the auxiliary pipes 5, 7 into an elliptic and saddle-like form. The requirement for the precision of this machining is severe in order to attain a close connection between the main pipes 1, 3 and the auxiliary pipes 5, 7. Accordingly, the machining work is difficult to conduct. Because of the elliptic and saddle-like form of the end surfaces of the branch pipes 5, 7, the welding line of the welding connection between the main pipes 1, 3 and the auxiliary pipes 5, 7 does not take the simple circular form. This non-circular welding line makes the welding work difficult. For these reasons, the production of the framework of the type described is not easy.
Furthermore, in the conventional framework in which the auxiliary pipes 5, 7 are welded directly to the main pipes 1, 3, there is a problem that, when a load P as shown in FIG. 1 is applied to the framework, a large local stress is generated in the juncture between the main pipes 1, 3 and the auxiliary pipes 5, 7 to cause a local deformation resulting in a deteriorated reliability in strength.
FIG. 4 shows another type of conventional framework using a joint 15. The joint 15 is constituted by a main part 9 and branching pipes 11, 13 projected from the main part 9, and is used in a manner shown in FIGS. 5, 6 and 7. Namely, auxiliary pipes 23, 25, 27, 29 and 31 are arranged between adjacent ones of three main pipes 17, 19 and 21 and are connected to the latter by the joints 15.
The part body 9 of the joint 15 has a semicylindrical form. In use of this joint, a complementary semicylindrical member 33 is united to this main part 9 by a butt welding to form a cylindrical body in combination with the latter. Then, the end surface of this cylindrical member, i.e. the end surfaces of the semicylinders 9 and 33 are welded to the end surface of the main pipe 17 so that the joint 15 is secured to the main pipe 17. Other joints 15 are secured to the main pipes 19 and 21 in the same manner. The end surface of any one of the auxiliary pipes 23, 25 and 27 is welded to the branch pipe 11 of the joint 15, while either one of the auxiliary pipes 29 and 31 is welded at its end to the branch pipe 13.
According to this arrangement, it is necessary only that the end surfaces of the auxiliary pipes 23, 25, 27, 29 and 31 are finished perpendicularly to the axes of the same auxiliary pipes, so that the processing is rather easy to perform. Also, the local deformation is avoided by adopting a sufficiently large plate thickness of the joint 15. However, the joint 15 having a sufficiently largely thickness can be produced only by a casting. If such a joint is to be produced by a plastic work, an extremely large press machine and special technic are required to make the production of the joint difficult and to increase the cost of the joint 15 uneconomically.
In addition, in the butt welding between the branch pipes 11, 13 of the joint 15 and the auxiliary pipes 23, 25, 27, 29, 31, as well as in the butt welding between the main part 9 and the main pipes 17, 19, 21, an impractically large bending stress is generated to reduce the reliability in strength if there is any misalignment of the pipes to be connected. Therefore, for assembling the framework structure with the joint 15, it is essential that the positioning of the members to be connected by butt welding be made at a high precision, resulting in a difficulty in the production of the framework.
In order to overcome the difficulty in the production of the joint 15 and the difficulty in the positioning of the joints 15 in relation to the main pipes 17, 19, 21 for butt welding, it is considered to form a joint 14 by welding to a main part 10 branch pipes 12 formed separately from the main part and superpositioning and welding the main part 10 to the main pipe 16 as shown in FIG. 8. This solution, however, poses a problem that the same difficulty as encountered in the welding of the auxiliary pipes to the main pipe as shown in FIGS. 1 and 2 is encountered in the welding of the branch pipes 12 to the main part 10 because the latter has a curved surface. In addition, the joint 14 formed by welding exhibits a reduced strength in the welded part as compared with the joint 14 formed as one body.
FIG. 9 shows a joint (4) used in a further different type of conventional framework, which is casted and has a spherically formed hollow portion to which auxiliary pipes are to be connected. FIG. 10 shows a segmentary view of a framework using this joint. According to this type, auxiliary pipes 6, 8, 10 are welded perpendicular to the surface of the spherical portion 2. Therefore, the auxiliary pipes 6, 8, 10 are sufficient to be machined perpendicular to its axis at their ends, and further are welded easily because the auxiliary pipes 6, 8, 10 contact always close to the surface of the spherical portion 12 inrespective of their fitting angles with respect to the main pipe. This type has, however, a deteriorated reliability in strength similar to the type shown FIG. 7, because misalignments between the main pipes 22, 24 and the joint 4 often occur in face to face welding, causing bending stress in the structure. To prevent these undesirable phenomenon, it is required to arrange the joint 4 and main pipes 22, 24 at their precise positions. This is a demerit that the production becomes rather difficult. Further, in the spherical portion 2 rather large local stress will be generated under a heavy loading on the auxiliary pipes, as similar as in the case shown in FIG. 3. In addition, under the heavy axial compression force on the main pipe 22, 24, the spherical portion 2 of the joint 4 is apt to buckle at the transition portion 26. Abovementioned defects cause a deteriorated reliability in strength.
The drawbacks common to both of conventional joints 14, 15 reside in that an excessive local stress is generated because the joints 14, 15 cannot smoothly transmit the stress between the main pipe and the auxiliary pipe due to the fact that the branch pipes 11, 12, 13 project at acute angles from the main parts 9, 10, and in that the strength is lowered due to the fact that the projecting ends of the branch pipes 11, 12, 13 wholly open. In consequence, the reliability in the strength of the framework structure using joints is lowered impractically.
Further, the framework of the type using the joint 4 has a poor productivity in assembly process, because it is required to arrange the joint 4 and main pipes 22, 24 at their precise position, and has a deteriorated reliability, because the spherical portion 2 of the joint 4 is apt to suffer local stresses and to buckle at the portion thereof to which the auxiliary pipes are welded.